Conductive layers deposited on a flexible transparent substrate have found major applications in various electronic and opto-electronic devices. A particular useful type of transparent conductive layer is based on metal oxides. These metal oxides can be applied to a substrate by different rather cumbersome methods. A review is given in K. L. Chopra et al, "Transparent Conductors--A Status Review", Thin Solid Films, 102, (1983), p. 1-46.
The most important metal oxides used in conductive layers are non-stoichiometric and doped oxides of tin, indium, cadmium, zinc and their various alloys. Well known examples of the latter category include tin oxide (TO) doped with antimony (ATO) or fluorine (FTO), indium oxide (IO) doped with tin (ITO), and zinc oxide doped with indium (IZO). These metal oxides exhibit high transmittance in the visible spectral region, high reflectance in the IR region and nearly metallic conductivity. The electrical as well as the optical properties of these materials can be tailored by controlling the deposition parameters.
Applications of these transparent conductive layers on flexible substrates can be divided in:
highly conductive layers (&lt;0.5 k.OMEGA./sq) with applications in displays (electroluminescent displays, Liquid Crystal Devices, Plasma Display Panels), touchscreens, solar cells and smart windows; PA1 conductive layers (&gt;0.5 k.OMEGA./sq) with applications in EMI-shielding foils, electroluminiscent lamps and membrane switches. PA1 in solar cells the use of TO layers is disclosed in JP-A 05-'218477, JP-A 05-218476, EP 290345, the use of FTO layers in JP-A 05-017878; PA1 for electroluminiscent displays the use of various metal oxide layers is disclosed in e.g. JP-A 09-024574, JP-A 08-281857, FR 2728082, JP-A 08-031572, and JP-A 01-081112; PA1 (a) preparing an aqueous medium containing at least one type of metal salt, PA1 (b) chemically reducing said metal salt by a reducing agent to form a dispersion of metal particles, PA1 (c) washing said dispersion of metal particles, PA1 (d) coating said washed dispersion onto a substrate, thereby obtaining a coated layer containing metal particles, PA1 (e) subjecting said coated layer to an oxidizing treatment to form a conductive layer containing metal oxide particles.
Other applications include heating elements for aircraft and automobile windows, heat-reflecting mirrors, antireflection coatings and gas sensors.
Examples of references dealing with the various applications of conductive layers based on metal oxides include:
A first main category of deposition techniques to form metal oxide layers on a substrate is evaporation. This technique can be further subdivided in post-oxidation of metal films, reactive evaporation, activated reactive evaporation, and direct evaporation. A second main category is sputtering, which can be further subdivided in reactive sputtering of metallic targets, direct sputtering of oxide targets and ion beam sputtering. Still further types of deposition techniques include reactive ion plating, chemical vapour deposition, spray pyrolysis, dip technique, chemical solution growth, reactive triode sputtering, and glow discharge composition.
In the scientific literature (M. Watanabe, Jpn. J. Appl. Phys., 9(1970) 1551; T. Nishino and Y. Hamakawa, Jpn. J. Appl. Phys., 9(1970) 1085; G. Bauer, Ann. Phys. (Paris), 30(1937) 433; G. Rupprecht, Z. Phys., 139(1954) 504; M. Watanabe, Jpn. J. Phys., 9(1970) 418) it is disclosed that a very thin (max. 50 nm) Sn layer (or In or Zn) deposited by evaporation can be oxidized whereby a transparent electrically conductive layer is formed. A serious drawback of this method is that with thicker layers the oxidation cannot be performed completely. Probably, due to the continuous phase nature of the metal film the passage of oxygen through this film is insufficient.